1. Field of the Invention
The invention relates to an apparatus and a method for the low-contamination melting of a substance in general and in particular for the melting of high-purity, aggressive and/or high-melting glass or glass-ceramic, specifically.
2. Description of Related Art
In traditional melting methods, glass is melted continuously in a platinum crucible or in refractory tank furnaces. A drawback of such methods is that some of the platinum is released to the melt, and the refractory tank furnaces have only short service lives. The desired high-purity glass quality cannot be achieved in this case.
There are also known methods in which glass is melted continuously in melting tank furnaces and removed. To obtain high-quality glass, a refining channel and a homogenization device or tank furnace may follow the melting tank furnace.
In both the above mentioned methods, discontinuous or continuous, the melting crucible or the melting tank furnace are externally heated, e.g. by a burner, and the heat is conductively transmitted to the melt in the interior. There is direct contact between the melt and the crucible or tank furnace. This has a number of drawbacks.
Firstly, the maximum melt temperature is limited by the crucible or tank furnace material. Therefore, the melting crucible or melting tank furnace and if appropriate the refining channel and the homogenization tank furnace usually consist of platinum, which has a relatively high melting point and is relatively resistant to corrosion.
Furthermore, the platinum melting crucible or the platinum melting tank furnace, and also the refining channel and the homogenization tank furnace, is attacked and corroded by the glass melt.
In any case, platinum disadvantageously leads to contaminations or impurities in the glass, which have an adverse effect on the optical properties, in particular the transmission, and consequently these conductive-heating methods can only be used to a very restricted extent for high-purity glasses. Impurities of this nature lead to transmission losses in optical fiber transmission systems of up to 200 to 500 dB/km.
This has proven extremely problematical in particular for the melting of aggressive glasses, e.g. zinc silicate or lanthanum borate glasses, since these glasses cause extensive corrosion to the crucibles or tank furnaces.
In addition to the conductive-heating methods mentioned above, it is also known to use methods in which glass is melted and heated inductively in a skull crucible.
A skull crucible typically comprises meandering water-cooled metal tubes which are spaced apart from one another. The melt inside the interior of the skull crucible is heated by a coil arrangement arranged around the skull crucible by high-frequency power being introduced into the melt.
Cooling of the skull crucible results in the formation of a substantially solid layer or crust of material of the same composition, i.e. in particular of glass, between the skull crucible and the melt. To this extent, impurities in the melt caused by the crucible material are significantly reduced.
A skull crucible is known, for example, from PETROV YU. B. et al., “Continuous Casting Glass Melting in a Cold Crucible Induction Furnace”, XV. International Congress on Glass 1989, Proceedings, Vol. 3a, 1989, pages 72 to 77.
However, the complex structure, in particular the high-frequency technology requirements of an inductively heated skull melting device, results in completely new requirements and problems compared to the abovementioned conductive-heating melting apparatuses. Firstly, the high melting temperature and very high throughtput rates per crucible volume means that many solution approaches for conductive-heating apparatuses cannot readily be transferred to skull melting apparatuses.
In principle, high melting rates and therefore high throughputs can be achieved with a skull melting apparatus. Although this is desirable, on the other hand this may under certain circumstances cause the quality of the melt and therefore of the end product to suffer, for example as a result of thermal reduction. This also leads to a deterioration in the transmission properties of the glass.
Furthermore, the rate at which the high-frequency radiation is introduced depends on various parameters of the melt. Therefore, the melting performance is restricted not only by the high-frequency power emitted by the coil arrangement but also by the melting parameters and crucible geometries.
Consequently, the known skull melting apparatuses are in need of improvement in particular with regard to the quality and homogeneity of the melt and also with regard to the melting capacity or throughput.
Document DE 199 39 780 A1 has disclosed an induction-heated skull crucible in which the metal tubes of the crucible wall are short-circuited with one another above the base, in order to displace the HF field upward or downward.
Document DE 199 39 779 A1 describes an apparatus for the continuous melting and refining of glasses and glass-ceramics, which comprises a melting vessel, a refining vessel and an agitation crucible. The agitation crucible is located at the end of a channel which is connected to the refining vessel.
Document DE 199 39 785 A1 has disclosed a method and an apparatus for producing colored glasses in which, during the further processing, the melt is fed through a skull device. This skull device is located downstream of a melting station.
Document WO 98/05185 A1 shows an induction furnace for glass melting, having a cooled tongue and an induction device beneath the tongue.
U.S. Pat. No. 3,244,494 has disclosed a method for introducing and melting in a glass furnace which is induction-heated. The result is a convective flow, but the flow rate of this convective flow is slow enough to ensure that raw material cannot or can scarcely descend into the melted glass.
The abovementioned apparatuses and methods can be improved further in terms of the melting capacity and glass quality.
WO 00/32525 has disclosed a method and an apparatus for vitrifying organic waste, in particular radioactive waste, in which the supply of oxygen used to oxidize the organic substances is effected both from the surface and from the underside of a melting crucible. Oxygen is supplied substantially in such a way that it has a locally limited influence. As a result, however, the redox state of the melt is only locally changed and the melt as a whole is not homogenized.